Split-gate trench power mosfet with protected shield oxide

ABSTRACT

A plurality of gate trenches is formed into a semiconductor substrate in an active cell region. One or more other trenches are formed in a different region. Each gate trench has a first conductive material in lower portions and a second conductive material in upper portions. In the gate trenches, a first insulating layer separates the first conductive material from the substrate, a second insulating layer separates the second conductive material from the substrate and a third insulating material separates the first and second conductive materials. The other trenches contain part of the first conductive material in a half-U shape in lower portions and part of the second conductive material in upper portions. In the other trenches, the third insulating layer separates the first and second conductive materials. The first insulating layer is thicker than the third insulating layer, and the third insulating layer is thicker than the second.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.15/062,912, filed Mar. 4, 2016, the entire contents of which areincorporated herein by reference. U.S. patent application Ser. No.15/062,912 is a divisional of U.S. patent application Ser. No.14/569,276, filed Dec. 12, 2014 (U.S. Pat. No. 9,281,368, issued Mar. 8,2016), the entire contents of which are incorporated herein byreference.

FIELD OF THE DISCLOSURE

Aspects of the present disclosure relate generally to semiconductorpower devices, and more particularly, to split-gate transistor devicesand methods of fabricating the same.

BACKGROUND OF THE DISCLOSURE

Power metal oxide semiconductor field effect transistors (MOSFETs) arecommonly used power devices due to their low gate drive power, fastswitching speed and superior paralleling capability. Most power MOSFETsfeature a vertical structure with source and drain regions on oppositesides of a gate trench filled with polysilicon as gate electrodes. Insuch structures, the MOS channels are formed along the vertical walls ofthe trenches.

In recent years, split-gate trench structures are developed and arepreferred for certain applications over conventional trench MOSFETsbecause they have good high frequency switching performance and lowon-state resistance. A split-gate trench power MOSFET comprises twoelectrodes in a gate trench. A first electrode serves as the gateelectrode to control the channel formation of the MOSFET, and a secondelectrode serves as shield electrode to decrease the capacitance C_(gd)between drain electrode and gate electrode. Existing fabricationtechniques for split-gate trench MOSFETs are typically complex andexpensive, usually requiring 8 or more masks to be applied duringprocessing.

It is within this context that embodiments of the present inventionarise.

SUMMARY OF THE PRESENT DISCLOSURE

It is therefore an aspect of the present disclosure to provide a new andimproved power MOSFET with a poly-protected shield oxide.

Briefly, aspects of the present disclosure include a semiconductordevice having a plurality of gate trenches formed into a semiconductorsubstrate in an active cell region and one or more other trenches formedinto the semiconductor substrate in a region other than the active cellregion. Each gate trench has a first conductive material in lowerportions of the gate trench and a second conductive material in upperportions of the gate trench. The first conductive material in the gatetrench is separated from the semiconductor substrate by a firstinsulating layer. The second conductive material in the gate trench isseparated from the semiconductor substrate by a second insulating layer,and separated from the first conductive material in the gate trench by athird insulating layer. Each of the one or more other trenches containspart of the first conductive material in a half U shape in lowerportions of the other trench and the second conductive material in upperportions of the other trench. The first conductive material and thesecond conductive material in the one or more other trenches areseparated by the third insulating layer. The first insulating layer isthicker than the third insulating layer, and the third insulating layeris thicker than the second insulating layer.

In some implementations, the semiconductor device includes one or morepickup trenches formed into the semiconductor substrate in a pickupregion. The pickup trenches contain at least part of the firstconductive material with the first insulating layer separating the partof the first conductive material in the one or more pickup trenches fromthe semiconductor substrate.

In some implementations, the each of the one or more other trenches haspart of the first insulating layer lining along bottom and at least onesidewall of the trench.

In some implementations, the one or more other trenches formed into thesemiconductor substrate in a region other than the active cell regionare peripheral trenches in a peripheral region, wherein the peripheralregion is provided between the active cell region and an edge of thedevice. In some implementations, the second conductive material in theperipheral trenches is separated from the semiconductor substrate by thesecond insulating material. In some implementations, each of theperipheral trenches has asymmetrical sidewall insulation with a firstinsulating layer on a side adjacent to the edge of the device and asecond insulating layer on a side adjacent to the active cell region.

In some implementations, the one or more other trenches formed into thesemiconductor substrate in a region other than the active cell regionare transitional trenches in a pickup region, wherein the transitionaltrenches are provided between the plurality of gate trenches and apickup trench. In some implementations, a part of the first conductivematerial in lower portions of the transitional trenches is in a U shape.In some implementations, a part of the third insulating layer in thetransitional trenches is in a U shape.

Another aspect of the present disclosure relates to a method forfabricating a semiconductor device. The method comprises a) forming aplurality of trenches by applying a first mask, the plurality oftrenches includes one or more gate trenches located in an active cellregion, and one or more transitional trenches and one or more pickuptrenches located in a pickup region; b) forming a first conductiveregion with a first conductive material in the plurality of trenches byapplying a second mask, wherein the gate trenches have their firstconductive region in lower portions of the gate trenches, the one ormore transitional trenches each has a U-shaped first conductive regionand the one or more pickup trenches each is filled with the firstconductive material; c) forming an intermediate dielectric region for atleast some of the trenches of the plurality, wherein the intermediatedielectric region for the one or more transitional trenches is in a Ushape; d) forming a second conductive region with a second conductivematerial in at least some of the trenches of the plurality; e) formingone or more body regions in the active cell region; f) forming sourceregions in the active cell region using a third mask; g) forming a firstelectrical contact to the second conductive region in the one or moretransitional trenches and to the first conductive region in the one ormore pickup trenches by applying a fourth mask; h) forming a secondelectrical contact to the second conductive region in the gate trenchesby applying a fifth mask; i) disposing a metal layer; and j) forming asource metal region and gate metal regions from the metal layer byapplying a sixth mask.

These and other aspects of the present disclosure will no doubt becomeobvious to those of ordinary skill in the art after having read thefollowing detailed description, which is illustrated in the variousdrawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top schematic diagram of a split-gate transistor deviceaccording to an aspect of the present disclosure.

FIG. 2 is a cross-sectional schematic diagram taken along lines I, IIand III of FIG. 1.

FIG. 3A-3H are cross-sectional schematic diagram taken along lines I, IIand III of FIG. 1 illustrating fabrication process of a split-gatetransistor device according to an aspect of the present disclosure.

DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 is a diagram illustrating a top view of a portion of a split-gatetransistor device according to an aspect of the present disclosure. Thesplit gate transistor device 100 of FIG. 1 includes a plurality ofsplit-gate array trenches 110 in the active cell region 101, aperipheral trench in the peripheral region 102 and a transitional arraytrench and a pickup trench in the pickup region 103. FIG. 2 is across-sectional schematic diagram taken along lines I, II and III ofFIG. 1. Specifically, in the active cell region 101, each split-gatetrench 110 has a bottom electrode 113 (i.e., shield electrode) and a topelectrode 115 (i.e., gate electrode). The bottom electrode 113 formed inthe bottom portion of the trench is electrically insulated from thesemiconductor substrate by a liner insulator material 112 (i.e., lineroxide or shield oxide), such as oxide or nitride, which may coat thewall of the split-gate trench 110 in which the bottom electrode 113 isformed. The top electrode 115 is formed in the top portion of thesplit-gate trench between the bottom electrode and a surface of thesubstrate 104. The top electrode 115 is separated from the semiconductorsubstrate 104 by an insulating material 116 (i.e., gate oxide), such asoxide or nitride and separated from the bottom electrode 113 by aninter-poly dielectric layer 114. In some implementations, the gate oxide116 has a thickness less than that of the inter-poly dielectric layer114, and the inter-poly dielectric layer 114 has a thickness less thanthat of the liner insulator 112.

A peripheral trench 120 is formed in the peripheral region 102. Theperipheral trench is lined with liner oxide 112 on the bottom and on thesidewall adjacent to the edge of the device and the bottom sidewalladjacent to the gate trench 110 and lined with gate oxide 116 along theupper sidewall adjacent to the gate trench 110. The gate electrode 115in the peripheral trench 120 is provided in the upper corner close tothe gate trench 110, and the shield electrode 113 is in a half U-shapeunder the gate electrode 115 with a half U-shaped inter-poly dielectriclayer 114 separating the gate electrode 115 and the shield electrode113.

A transitional trench 130 and a pickup trench 140 are formed in thepickup region 103. The transitional trench 130 has a liner insulator112, e.g., an oxide, along sidewalls and bottom of the trench. The gateelectrode 115 in the transitional trench 130 is provided in the middleupper portion of the trench and the shield electrode 113 is in a U shapewith the gate electrode 115 nested within an opening of the U shape. AU-shaped intermediate dielectric layer 114 separates the gate electrode115 and the shield electrode 113. The pickup trench 140 contains ashield electrode 113 with a liner insulator 112 along sidewalls andbottom of the trench.

FIG. 3A-3H are cross-sectional schematic diagram taken along lines I, IIand III of FIG. 1 illustrating fabrication process of a gate transistordevice according to an aspect of the present disclosure. An N-typedevice is described for purposes of illustration. It should be notedthat P-type devices may be fabricated using a similar process but withthe opposite conductivity type. In FIG. 3A, an N-type substrate 104(e.g., an N+ silicon wafer with an N− epi layer grown on it) is used asthe drain of the device. In some embodiments, the doping concentrationfor the upper portions of the substrate 104 is approximately1×10¹⁶-1×10¹⁷/cm³, with thickness of 2-4 μm. A hard mask layer can beformed on top of the substrate 104, for example, by forming a thin oxidelayer 105 on the substrate 104 by deposition or thermal oxidation,followed by a nitride layer 106 on top of the thin oxide layer 105. Insome embodiments, the thickness of the silicon oxide layer 105 rangesfrom about 100 Å to 500 Å and is preferably about 200 Å. In someembodiments, the thickness of the nitride layer 106 ranges from 1500 Åto 4000 Å and is preferably about 3500 Å. Another oxide layer 107 may bedisposed on top of the nitride layer 106 to form the hard mask with anoxide/nitride/oxide stack. In some embodiments, the thickness of theoxide layer 107 ranges from 1000 Å to 3000 Å and is preferably about2000 Å. A photo resist layer (not shown) is then applied on top of theoxide/nitride/oxide stack and patterned using a trench mask. A hard masketch is then performed to etch away exposed portions of the nitridelayer 106 and oxide layers 107 and 105 and the etching stops at thesilicon surface leaving openings that will be used to mask the etchingof trenches. The trench openings are etched into the semiconductorsubstrate 104 forming gate trenches 110 in active cell region 101, aperipheral trench 120 in the peripheral region 102 and a transitionaltrench 130 and a pickup trench 140 in the pickup region 103. In someembodiments, the target depth of these trenches is approximately 0.6-1.5μm, preferably about 1.0 μm. Regions 111 of the substrate between thegate trenches 110 or between a gate trench 110 and the peripheral trench120 are sometimes referred to herein as mesa regions.

In FIG. 3B, an oxidation is performed to grown a liner oxide layer 112.A thick liner insulator layer 112 is grown on the trench walls to athickness based on device optimization for low R_(ds) and high breakdownvoltage. In some embodiments, the thickness of the liner insulator layer112 is about 400-800 Å and preferably 600 Å. Next, a first conductivematerial 113, such as polysilicon, is deposited into the trenches andover the semiconductor substrate 104 followed by a chemical mechanicalpolishing (CMP) to remove polysilicon over the oxide layer 107 leavingthe conductive material only in the gate trenches 110, the transitionaltrench 130 and the pickup trench 140. The first conductive materiallayer 113 in the trenches 110, 130 and 140 is then etched back and theetching stops at 100-600 Å below the surface of the substrate 104, e.g.,about 300 Å below the surface of the substrate 104. This first layer ofconductive material 113 is sometimes referred to as source poly, shieldpoly or Poly 1.

With reference to FIG. 3C, a photo resist layer 150 (e.g., poly 1 mask)is applied on top of portions of the oxide/nitride/oxide stack to coverthe pickup trench 140, two sides of the transitional trench 130 and oneside of the peripheral trench 120 that is adjacent to the edge of thedevice. The exposed first conductive material 113 is then etched back inthe upper portions of the gate trenches 110 and the peripheral trench120 and the transitional trench 130 as shown in FIG. 3C. The photoresistlayer 150 is patterned in a way that protects the liner insulator 112and at least a portion of the first conductive material 113 in theperipheral trench 120, the transitional trench 130 and the pickup trench140 as the etching leaves a half U-shaped conductive material 113 in theperipheral trench 120 and a U-shaped conductive material 113 in thetransitional trench. In one example, the first conductive material layer113 may be etched to a target depth using a timed etch-back process. Insome embodiments, the conductive material layer 113 is etched to a depthabout 0.55 μm below the surface of the semiconductor substrate 104.

With reference to FIG. 3D, the exposed liner insulator 112 along theetched upper portion of the gate trenches 110 and the peripheral trench120 is stripped, e.g., using a wet etch. A thin insulating layer 116(e.g., gate oxide) is formed to cover the upper portion of trench wallsof the gate trenches 110. In addition, the upper portion of the trenchwall next to the gate trench 110 of the peripheral trench 120 is alsolined with the gate insulator 116. Next, an intermediate dielectriclayer 114 is formed atop the bottom portion of the first conductivematerial layer 113 of the split-gate trenches 110. The intermediatedielectric layer 114 is also formed along the conductive material layer113 in the peripheral trench 120 and the transitional trench 130 (e.g.,by oxidation) such that the intermediate dielectric layer 114 is in ahalf U shape in the peripheral trench 120 and in a U shape in thetransitional trench 130. The gate oxide 116 is about 150 to 500 Å inthickness and the inter-poly dielectric layer 114 is about 250 to 800 Åin thickness.

A second layer of conductive material 115, such as polysilicon isdeposited into upper portion of the trenches 110, 120 and 130 and overthe substrate 104 followed by a CMP to remove polysilicon over the oxidelayer 107 and the photo resist layer 150. In the particular case ofpolysilicon, this second layer of conductive material 115 is sometimesreferred as gate poly or Poly 2. The second conductive layer 115(sometimes referred to herein as the gate conductor) is then etched backto about 100 Å-600 Å, e.g., about 300 Å below the surface of thesubstrate so that the upper surface of the second conductive material inthe trenches 110, 120 and 130 is recessed below the top surface of thesemiconductor substrate 104. After removing the photo resist 150,another oxidation step is performed in the trenches 110, 120, 130 and140 forming the insulating layer 152 followed by a CMP to remove oxides152 and oxide layer 107 above the nitride layer 106.

With reference to FIG. 3E, the nitride layer 106 is stripped using a wetetch. A body dopant implant may then be performed to form a plurality ofbody regions. This implant may be carried out, e.g., by a blanketimplant followed by a body diffusion to form the body region 154. Thebody drive diffuses the dopant to a desired depth. Then a source mask isapplied to carry out a source dopant implant to form a plurality ofsource regions 156 in the active cell region 101. A layer of nitride isdeposited over the top of the structure. In one example, the thicknessof the nitride layer is about 600 to 1200 Å, e.g., about 900 Å. Thenitride layer is anisotropically etched back along the horizontalsurface to form nitride spacers 157 along the walls next to the oxides152. Following that, a stop layer of nitride 158 is deposited on top ofthe structure. In one example, the thickness of the nitride stop layer158 is about 200 Å to 500 Å, e.g., about 300 Å. Next a low temperatureoxide (LTO) layer and a borophosphosilicate glass (BPSG) layer 159 aredeposited as shown in FIG. 3E.

With reference to FIG. 3F, a first contact mask is applied. In thepickup region 103, an etch step is performed to etch ONO stack (LTOlayer 159, nitride stop layer 158, oxide 152) to reach the surface ofthe top conductive material (i.e., the conductive material 115 in thetransitional trench 130 and the conductive material 113 in the pickuptrench 140) forming contact openings 155S and 155G through the LTO layer159 to allow for contact to the shield conductor 113 and gate conductor115, respectively, at regions II and III.

In FIG. 3G, another contact mask 160 is applied and another etch processis performed to open the arrays of source and body contact openings inthe mesa regions 111 between the gate trenches 110 in the active cellregion 101. Specifically, the etching process starts with removal theLTO layer 159 and stops at the nitride stop layer 158. Following abreakthrough of nitride 158, the contact trenches 162 are formed.

Next, a barrier metal (not shown) is lined on the sidewalls and bottomof the contact openings 155S and 115G and contact trenches 162 followedby the deposition of a conductive material, e.g., tungsten, in thecontact openings and contact trenches forming the conductive plugs 164.A layer of metal is deposited. A metal mask is used and an etch processis performed to form a source metal region 166 and gate metal regions168 and 170. The conductive plugs 164 are formed in the connected to themetal regions as shown in FIG. 3H. The wafer is then followed the restof standard trench MOSFET steps to complete the processing.

The fabrication process according to the present disclosure allows thethick liner insulator 112 to be preserved using one less mask. It doesnot need a P-cover mask to protect the liner oxide 112 in the peripheraltrench 120. This makes the process less expensive. The conductivematerial 113 that forms the shield electrode protects the linerinsulator 112 during the insulator etch process that forms openings forcontacts to the shield conductor 113 and the gate conductor 115.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. For example, althoughn-channel devices are described above, aspects of the present disclosurecan be implemented as p-channel devices as well simply by reversing theconductivity types of the doped regions described above. Variousalterations and modifications will no doubt become apparent to thoseskilled in the art after reading the above disclosure. Accordingly, itis intended that the appended claims be interpreted as covering allalterations and modifications as fall within the true spirit and scopeof the invention.

We claim:
 1. A semiconductor device, comprising: a plurality of gatetrenches formed into a semiconductor substrate in an active cell region,each gate trench having a portion of a first conductive material inlower portions of the gate trench and a portion of a second conductivematerial in upper portions of the gate trench; and one or moretransitional trenches formed into the semiconductor substrate in apickup region, each transitional trench having a portion of the firstconductive material in lower portions of the transitional trench and aportion of the second conductive material in upper portion of thetransitional trench, wherein the transitional trenches are providedbetween the plurality of gate trenches and one or more pickup trenchesformed into the semiconductor substrate in a pickup region, wherein theportion of the first conductive material in the lower portions of thetransitional trenches in a U shape, wherein the portions of the firstconductive material in each gate trench and each transitional trench areseparated from the semiconductor substrate by a first insulating layer,and wherein the portions of the second conductive material in each gatetrench and each transitional trench are separated from the semiconductorsubstrate by a second insulating layer and separated from the portionsof the first conductive material in each gate trench and eachtransitional trench by a third insulating layer.
 2. The device of claim1, wherein the semiconductor substrate is an N-type semiconductorsubstrate.
 3. The device of claim 1, wherein the semiconductor substrateis a P-type semiconductor substrate.
 4. The device of claim 1, whereinthe one or more pickup trenches are connected to the one or more gatetrenches, wherein the one or more pickup trenches contain at least partof the first conductive material with the first insulating layerseparating the part of the first conductive material in the one or morepickup trenches from the semiconductor substrate.
 5. The device of claim1, wherein each of the one or more transitional trenches has part of thefirst insulating layer lining along its bottom and at least one of itssidewalls.
 6. The device of claim 1, further comprising one or moreperipheral trenches formed into the semiconductor substrate in aperipheral region other than the active cell region, wherein theperipheral region is provided between the active cell region and an edgeof the device.
 7. The device of claim 6, wherein the part of the secondconductive material is in the peripheral trenches and is separated fromthe semiconductor substrate by the second insulating material.
 8. Thedevice of claim 6, wherein each of the peripheral trenches hasasymmetrical sidewall insulation with a first insulating layer on a sideadjacent to the edge of the device and a second insulating layer on aside adjacent to the active cell region.
 9. The device of claim 1,wherein a part of the third insulating layer in the transitionaltrenches is in a U shape.
 10. The device of claim 1, wherein the firstinsulating layer is thicker than the third insulating layer, and thethird insulating layer is thicker than the second insulating layer. 11.A method for fabricating a semiconductor device, comprising: forming aplurality of gate trenches into a semiconductor substrate in an activecell region, each gate trench having a portion of a first conductivematerial in lower portions of the gate trench and a portion of a secondconductive material in upper portions of the gate trench; and formingone or more transitional trenches formed into the semiconductorsubstrate in a pickup region, each transitional trench having a portionof the first conductive material in lower portions of the transitionaltrench and a portion of the second conductive material in upper portionof the transitional trench, wherein the transitional trenches areprovided between the plurality of gate trenches and a pickup trench,wherein the portion of the first conductive material in the lowerportions of the transitional trenches in a U shape, wherein the portionsof the first conductive material in each gate trench and eachtransitional trench are separated from the semiconductor substrate by afirst insulating layer, and wherein the portion of the second conductivematerial in each gate trench and each transitional trench are separatedfrom the semiconductor substrate by a second insulating layer andseparated from the portions of the first conductive material in eachgate trench and each transitional trench by a third insulating layer.12. The method of claim 11, wherein the first insulating layer isthicker than the third insulating layer, and the third insulating layeris thicker than the second insulating layer.
 13. The method of claim 11,further comprising forming one or more pickup trenches connected to theone or more gate trenches, wherein the one or more pickup trenchescontain at least part of the first conductive material with a firstinsulating layer separating the part of the first conductive material inthe one or more pickup trenches from the semiconductor substrate. 14.The method of claim 13, wherein each of the one or more gate trenches,one or more pickup trenches, and one or more other trenches, has part ofthe first insulating layer lining a bottom and at least one sidewall ofthe trench.
 15. The method of claim 13, wherein the one or more othertrenches include a trench located between the active cell region and anedge of the device.
 16. The method of claim 11, further comprisingforming one or more peripheral trenches in a peripheral region betweenthe active cell region and an edge of the semiconductor device, whereinthe one or more peripheral trenches each has a half U-shaped firstconductive region.
 17. The method of claim 16, wherein the part of asecond conductive material in the peripheral trenches is separated fromthe semiconductor substrate by the second insulating material.
 18. Themethod of claim 17, wherein each of the peripheral trenches hasasymmetrical sidewall insulation with the first insulating layer on aside adjacent to the edge of the device and a second insulating layer ona side adjacent to the active cell region.
 19. The method of claim 11,wherein a part of an intermediate dielectric region in the othertrenches is in a U shape.